Redstone Coherer

A glass tube of loose redstone grains. It starts open. A nearby spark welds the grains together and it turns into a piece of wire, and it stays one until you tap it. It is the first detector that doesn't need a wire running to what it listens for.

Crafting

G G G
W R W

G is Glass, W is Copper Wire and R is Redstone.

Using it

Place it in a line like a cable. Open, it splits the line in two; welded, it joins them. Whatever is on the far side (a Relay's electromagnet, a load, a Current Sensor linked to a Controller) is what the detection turns on.

• Right-click with an empty hand to tap it: "Coherer tapped: grains loose, open".
• A Controller can tap it with a Switch card by switching it off. It can't switch it on: only a spark does.

What a spark is

An arc: a Breaker or Relay opening with at least 1 A flowing, or a turn of an Electrostatic Generator. It carries 16 blocks. Your own breakers count, so a coherer beside one that flaps under load will weld itself.

Left welded

A welded coherer passes everything, like any wire. Forgetting to tap it is the usual failure.

Overrun

More than 2 A through a welded coherer sinters the grains. Its condition falls (to 20%), and a tap then frees it only some of the time: at 20% condition, one tap in five. It never stops working, you just tap more. Welding and tapping cost nothing; only the overload wears it.